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<script>
    function strConvert(str) {
        let str1 = str.split("")
        for (let i = 0; i < str1.length; i++) {
            if (str1[i].charCodeAt() > 65 && str1[i].charCodeAt() <= 90) {
                str1[i] = str1[i].toLowerCase();
            } else {
                str1[i] = str1[i].toUpperCase();
            }
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        return str1.join("")
    }
    console.log(strConvert("helloWORLD"))
</script>